Two events hint at impact of dark matter particles

Researchers are reporting that a detector buried in a Minnesota mine may have …

There's a chance that today will go down as the day of the first announcement that we've detected the presence of particles of dark matter. Two talks from members of the CDMS consortium, which runs a detector designed to spot the presence of a likely dark matter candidate, have indicated that they've spotted two events that bear the signatures of something called a neutralino, a hypothesized particle that has many of the properties of dark matter. With only two of these detections, however, there's still a 23 percent chance that random background events produced the signals.

We've been looking for dark matter for quite a while, as its presence was suggested by simple observations: lots of things in the Universe behave as if there's more gravitational attraction than the visible matter could possibly produce. That suggests that either our understanding of gravity's wrong, or there's matter we can't see.

As the observational evidence for dark matter has piled up in the intervening years, scientists have gradually whittled down the list of potential candidates. What started as a fight between MACHOS and WIMPS—massive compact halo objects (black holes, brown dwarfs, and the like) and weakly interacting massive particles—eventually left WIMPS as the last candidate standing. Our measurements of the cosmic background radiation from the big bang indicates not only that dark matter is the majority of the matter out there, but that the dark material isn't likely to be normal, baryonic matter. That leaves known astronomical objects out.

With normal matter out of the picture, the search turned towards the particle zoo predicted by the Supersymmetric Standard Model, which served up something called the neutralino X01 (the lightest of a series of hypothetical particles) as a promising candidate. It's quite heavy, and only interacts with normal matter very weakly. But the key thing is that it should interact on rare occasions, which raises the prospect that the interactions could be detected.

That's what prompted the Cryogenic Dark Matter Search I and its successor, CDMS II, which have been using a mix of high- and low-tech to search for a likely dark matter candidate. The low tech involves the method for shielding its detectors from confounding signals from cosmic rays: they're buried at the bottom of a mine in Minnesota.

The high tech involves the detectors, which need to be sensitive enough to detect a neutralino bumping into the nucleus of an atom as it glides through Earth. The energy imparted by these collisions is going to be very subtle, in the area of 10keV, so the atoms in the detector have to be kept still enough for it to stand out. That's achieved by chilling them down to about 10 milliKelvin. At that temperature, even the subtle nudge of a neutralino will shift the atomic lattice in a way that alters current in a neighboring superconductor, allowing the impact to be registered. (Those wanting the gory details may want to check the CDMS team's explanation).

The first run came up empty, and the results were published back in 2004. But everyone involved had indicated they learned something in the process of generating the null result, and the detectors continued to run. Then, about two weeks back, rumors started circulating that there'd be a major announcement from CMDS. Last week, the team announced that its data analysis from run II was complete, and there would be two talks today, one at Stanford, one at Fermilab.

Jodi Cooley in front of a slide showing the two events that may represent the detection of dark matter particles.

SMU's Jodi Cooley gave the Stanford talk, and she started by providing a longer and more informative version of the background I've just given, describing how they reject false positive events (eliminate those on the detectors' surface, and check whether they are ionizing events). They then did Monte Carlo modeling of a null result, and compared that to the data obtained with the detector. All in all, a grand total of two events stood out; a third was just outside of their range. The probability of observing these events from a source other than dark matter is about 23 percent.

In and of itself, it's obviously not a demonstration that we've detected dark matter particles. But the results certainly suggest that we could be on the right track, and the CDMS team is heading back to the mines for more. There's already talk of building a deeper detector, and the existing location will be upgraded with additional detectors prior to its next run. Each additional event that survives through the filtering will improve our overall confidence in the earlier detections; one source indicates that we'd need less than 10 total detections within the CDMS' range in order to have a high degree of confidence in the results.

It's actually an exciting time for dark matter research. There have been unusual observations that suggested astronomers might be getting a handle on it, and the LHC was expected to reach energies that might produce some of the WIMP candidates, so the whole thing is taking on the feel of a race. It may take another couple of years for CDMS to pick up enough signals, so everyone else involved knows the clock is ticking.